Tweezer probe and arm therefor

ABSTRACT

A tweezer probe comprises a pair of arms (10) each made of a multi-layer printed circuit board. The outer conductive layers (20, 32) on each arm are etched to form a shield region (36) and a tip contact (34). The tip contacts (34) are connected to inner conductive layers (24, 28) by through-hole plating, the inner layers being etched to form connector wires (40) extending along the length of the arms to a connector region where leads (8) are attached. The arms (10) are substantially rigidly mounted together, with an adjustment mechanism (14, 72, 74, 76) being provided to alter the angle between the arms, the tips being moveable towards each other against the bias provided by the inherent resilience of the arms.

This application is a division of application Ser. No. 08/674,409, filedJul. 2, 1996, which is a continuation of application Ser. No. 08/418,307filed on Apr. 7, 1995 abandoned.

This invention relates to measurement probes, and particularly tweezerprobes which consist of a pair of arms having conductive tips which canbe squeezed s together against a bias so as to bring the tips intocontact with a circuit element to be tested. Each tip is connected to arespective lead, or in some known versions the tip is split to form tworegions each connected to a respective lead, both regions being in usebrought into contact with the same test point so that measurements canbe made using the Kelvin contacts technique.

However, tweezer probes are made from a large number of precisionmoulded parts which are expensive to manufacture and assemble. It isdifficult to avoid lateral displacement of the arms which causes thetips to become misaligned; this results in particular problems withsplit tips.

A further problem arises in that the capacitance characteristics of thetweezer probe may change as the probe arms are moved towards each otherdue to the changing distance between the connecting wires within theprobe arms.

According to one aspect of the invention there is provided an arm for atweezer probe, the arm being formed of a printed circuit board having aconductive tip portion on a face thereof, and a connecting portionformed of a conductive layer extending from the tip portion to aconnector region for attachment to a lead.

The invention thus extends to a tweezer probe comprising a pair of armsaccording to the first-mentioned aspect of the invention. It is proposedthat the tweezer probe should comprise a pair of such arms mountedtogether with their tip portions facing each other. Preferably, themounting is substantially rigid, and the tip portions can be movedtogether against the bias provided by the inherent resilience of thearms.

Preferably, the connecting portion is formed by etching of an innerconductive layer of a multi-layer board, the tip portion preferablybeing formed by etching of a outer layer. Preferably, the majority ofthe outer layer is left as shielding. There may be a corresponding tipportion and shielding area on the opposite side of the arm. A further,inner conductive layer may be provided to form an additional connectingportion. The tip portion or portions can be connected to the connectingportion or portions by through-hole plating.

By forming a tweezer probe arm from a printed circuit board, it ispossible to make the arm much more easily and cheaply than in prior arttweezers. Furthermore, assembly of the probe is made easier because itis possible to rely on the inherent resilience of the arms for biassing,rather than providing a separate spring or the like. This in turn allowsa substantially rigid mounting of the arms, which inhibits lateralrelative movement of the tips so that there is less chance ofmisalignment. Such misalignment is also inhibited by the fact that thearms are substantially planar, and therefore cannot easily be deformedabout an axis perpendicular to their planes.

In accordance with a further aspect of the invention there is provided atweezer probe comprising a pair of resilient arms each having aconductive tip portion connected to at least one respective lead, andmeans remote from the tip portions for substantially rigidly mountingthe arms together, whereby the tip portions can be moved towards eachother against the bias provided by the inherent resilience of the arms,the mounting means including adjustment means for adjusting the relativerest positions of the tip portions.

Thus, the arrangement is facilitated by not requiring a separatebiassing means, but any problems due to the limited range of movement ofthe tips are avoided by using the adjustment means to alter this range.

An arrangement embodying the invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a side elevation of a tweezer probe in accordance with theinvention and the connecting lead therefor;

FIG. 2 is a plan view of the tweezer probe;

FIG. 3 is a perspective view of the tip portion of one of the arms ofthe tweezer probe;

FIGS. 4A, B and C show conductive areas of respective layers of the arm,FIG. 4C illustrating a modification of the embodiment; and

FIG. 5 is a perspective view schematically showing the mounting assemblyof the tweezer probe.

Referring to FIGS. 1 and 2, a tweezer probe 2 is connected by a cable 4to a standard connector 6, the cable 4 having individual leads 8extending therefrom for connection to the respective arms 10 of theprobe. The arms are fixed together by a mounting assembly 12 which isdescribed in more detail later. The assembly 12 has a pair of adjustmentwheels 14 disposed one on each side thereof, which can be rotatedtogether to adjust the relative angular rest positions of the arms 10between the positions shown in broken lines at 16 and the positionsshown in broken lines at 18.

Each of the arms 10 has a conductive tip, the tips being connected tothe wires 8. The arms can be squeezed together to bring the tips intocontact with a circuit element to be tested. The arms are squeezedagainst the bias provided by their inherent resilience. This provideseffective operation over a limited range of tip separations, but bymaking a preliminary adjustment using the wheels 14, this range can beadjusted to suit the circuit element being tested.

Each arm is formed of a multi-layer printed circuit board. Referring toFIG. 3, the circuit board comprises an upper conductive layer 20, afirst insulating layer 22, a conductive layer 24, a second insulatinglayer 22, another conductive layer 2 8, a third insulating layer 30 anda lower conductive layer 32 which is not visible in FIG. 3 but whichforms the opposite face of the arm 10. In a particular embodiment theinsulating layers 22 and 30 are each formed of a pair of face-to-facelayers. As an alternative, the arm could be formed of two separatedouble-sided printed circuit boards affixed face-to-face, possibly withan intervening insulating layer, or one double-sided board and onesingle sided board affixed face-to-face so that there are two externaland one internal conductive layers.

The multi-layer printed circuit board is manufactured in a per se knownmanner, with each of the conductive layers 20, 26, 28 and 32 beingetched to a desired configuration. The layer 20 which in use forms theinner face of the tweezer arm 10 is etched as shown in FIG. 4A to form adiscrete tip portion 34. Substantially all the rest of the layer is leftto form a shield area 36. At the end remote from the tip portion 34, twocircular contact portions 38 are formed, each separated from the shieldarea by a circular gap. These contact portions are intended forconnection to respective ones of the wires 8.

The layer 32 on the opposite side of the arm 10 is etched in a generallysimilar configuration.

Each of the inner conductive layers 24 and 28 is etched as shown in FIG.4B. The etching forms two wires 40 each of which is connected to a tipcontact pad 42 at one end and a respective lead contact pad 44 at theother end, the lead contact pads 44 being aligned with the contact pads38 of the layers 20 and 32, and the tip contact pad 42 being alignedwith the tip portions 34 of the upper and lower layers 20 and 32. Thereare also circular conductive portions 46 surrounding areas in whichmounting holes for the arms will be drilled.

During assembly of the arms 10, a hole is drilled through the tipportions 34 of the layers 20 and 32 and the tip contact pads 42 of theinner layers 24 and 28, and these conductive regions are allinterconnected by through-hole plating. Similarly the aligned contactpads 38 and 44 of the layers 20, 24, 28 and 32 are interconnected bythrough-hole plating.

Referring to FIG. 3, metal, e.g. gold, is deposited on the edge of theprinted circuit board in the tip region, e.g. by electroless plating,the metal contacting the tip portions 34, as shown at 48. When using theprobe arm, therefore, the arm can be applied to a test point usingeither the inner or outer faces or the surrounding edge portion of thetip.

FIG. 5 shows the mounting assembly 12. This primarily comprises an upperplate 50, an upper body 52, a lower body 54 and a lower plate 56. Thesemay all be formed of, e.g. machined metal. One of the arms 10 issandwiched between the upper plate 50 and the upper body 52, these beingsecured together by two screws 58 (see FIG. 2) located at positionsshown by lines 60 (for clarity these screws and the associated holesbeing omitted from FIG. 5). Similarly, the lower arm 10 is securedbetween the lower plate 56 and the lower body 54, by means of screwslocated at positions indicated by lines 62. By securing each arm at twospaced locations, the arm is held more rigidly against movement about anaxis normal to the plane of the arm, which might otherwise tend to causemisalignment of the tip portions.

The upper body 52 has a downwardly-projecting extension 64 which ispositioned alongside an upwardly-directed extension 66 of the lower body54. The extensions 64 and 66 have aligned holes 68 and 70 which receivea pivot pin (not shown) which secures the bodies 52 and 54 together butallows pivotal movement about an axis which is substantiallyperpendicular to the length of the arms 10, and disposed in a planeparallel to the planes of the arms.

The extension 64 has a circular hole 72 which is aligned with anon-circular hole 74 in the extension 66, these holes being arranged toreceive a shaft 76 interconnecting the adjustment wheels 14 (only one ofwhich is shown in FIG. 5). The shaft 76 has a circular section part 78which locates in the hole 72 and an eccentric part 80 which locates inthe hole 74. It will be appreciated that rotation of the wheels 14causes the eccentric part 80 to rotate within the non-circular hole 74,thereby altering the spacing between the upper and lower bodies 52 and54 at a position spaced from the axis of the pivot pin. Thus, the anglebetween the bodies 52 and 54, and therefore between the arms 10, isadjusted.

The bodies 52 and 54 are provided with recesses (not shown) in,respectively, their upper and lower faces, so that each body can receivea pair of wires 8, each of which is connected by soldering or pressurecontact to a respective one of the contact pads 38 formed on the layer20 of the respective arm 10.

If desired, each tip portion 34 can be split into separate regions asshown in FIG. 4C, each region 34a, 34b being connected to a respectiveone of the contact wires 40 in each layer 24 and 28, so thatmeasurements can be made using the Kelvin contact technique. Preferably,the two areas are interdigitated as shown, with only a small gaptherebetween, so that simultaneous contact with a circuit element can bemade more reliably.

What we claim as our invention is:
 1. A tweezer probe, comprising:a pairof elongate arms each formed of a printed circuit board having aconductive tip portion disposed on a surface thereof at one end of eachof said arms, a connector region for attachment to a lead spaced apartfrom conductive tip portion, and a conductive layer forming a connectingportion extending from said tip portion to said connector region; andmounting means spaced from said tip portions of said arms for mountingsaid arms together substantially rigidly, so that said tip portions canbe squeezed toward each other against bias provided by inherentresilience of said arms, wherein said mounting means has adjustmentmeans for adjusting the relative positions of said tip portions.
 2. Atweezer probe in accordance with claim 1 wherein said adjustment meansis operable to alter the angle between said pair of arms.